Since ZK-3225 hidden cardlayout children are now invisible in the dom, and become visible if necessary. Only the child at the selected index remains visible.
The visibility is currently triggered directly via CSS.
This direct visibility change has a sideffect that nested widgets are unaware of being shown/hidden.
e.g. The gmaps component needs an indication that it is being shown to render properly.
(maybe other components are affected as well)

An alternative could be to use the widget API methods show()/hide() which will notify the children of a visibility change accordingly preventing this issue.

Attached a reproducing example: cardlayout-tabbing-issue-gmaps.zul